1. Field of the Invention
This invention relates to hydrogen storage alloys useful as a material for transportation and storage of hydrogen, and more particularly to magnesium-based hydrogen storage alloys having a large hydrogen storage quantity at a lower temperature under a lower pressure.
2. Description of Related Art
Recently, a hydrogen energy is expected as a petroleum alternative energy. However, there are left many problems to be solved such as infrastructure building, safety and the like for putting the hydrogen energy into practice.
When hydrogen is filled into a high pressure steel bottle, a volume can be compressed to about 1/150 or to about 1/800 at a liquefied state. On the other hand, hydrogen storage alloys can occlude (absorb) a gaseous hydrogen therein and store as a solid, so that a volume of hydrogen can be apparently compressed to about 1/1000. Furthermore, the hydrogen stored in the hydrogen storage alloys is simple in the handling as compared with a liquid hydrogen or a high pressure hydrogen gas. Therefore, there is a great merit in practical use when hydrogen is transported and stored by using the hydrogen storage alloys, and it is advantageous in the safety because it is not required to handle hydrogen as a high pressure gas or a liquid.
And also, hydrogen can be absorbed or desorbed only by adjusting a temperature or a pressure in the hydrogen storage alloys, so that it is possible to build a cheap hydrogen storage equipment by using the hydrogen storage alloys and it is attempted to reduce energy cost.
From the above reasons, or from a viewpoint of a future energy agenda, it is urgently demanded to develop hydrogen storage alloys for transporting and storing hydrogen in a high performance.
As the conventionally discovered hydrogen storage alloys, there are mainly known binary intermetallic compounds of AB5 type such as LaNi5 or the like, AB2 type such as ZrMn2 or the like, AB type such as TiFe or the like, and A2B type such as Mg2Ni or the like. Among them, a hydrogen storage quantity of the hydrogen storage alloys other than Mg2Ni is as small as about 1.4 mass % in LaNi5, about 1.7 mass % in ZrMn2, and about 1.8 mass % in TiFe based on the weight of the alloys, which is mainly composed of a relatively heavy element such as a rare earth element or Zr, so that it is difficult to increase the hydrogen storage quantity per the weight, and V and the like are expensive and are costly less in the merit.
On the other hand, Mg2Ni as a typical example of A2B type alloy is about 3.6 mass % in the hydrogen storage quantity, which is considerably larger than that of the other alloys. However, it is demanded to develop a hydrogen storage alloys with a higher capacity. Particularly, hydrogen storage alloys mainly composed of magnesium are known to have a very large hydrogen storage performance. Moreover, a hydrogen storage quantity of magnesium metal itself (H2/(H2+Mg)) reaches 7.6 mass %.
However, the magnesium-based hydrogen storage alloys are not put into a practical use up to the present time. Because, it is considered to be difficult to initially activate the magnesium alloys. That is, magnesium is easy to absorb hydrogen and to form a stable hydride, so that it is required to maintain at high temperature and high pressure state of 350-450° C. and 10-20 MPa for absorbing and desorbing hydrogen, which becomes difficult to put into the practical use as a hydrogen storage alloy.